Automatic way to increase zone file serial number by one in BIND9

This would be pretty difficult to do in 'sed' but it is fairly easy in 'awk'.

 awk '{ if ( $0 ~ /[\t ]SOA[\t ]/ ) $7=$7+1; print}'

If it is an SOA line ("SOA" with a space or tab before and after it) then increment field 7 (serial number). Print every line.

This only increments the serial number which is what was asked, however you'll still need to trigger a reload and notifies to the secondary servers to propagate the changes.